Method of producing secondary battery

ABSTRACT

A method of producing a secondary battery disclosed here includes forming a positive electrode active material layer containing a lithium- and manganese-containing composite oxide on a positive electrode current collector to produce a positive electrode; measuring a peel strength between the positive electrode active material layer and the positive electrode current collector; producing a secondary battery assembly including the positive electrode, a negative electrode, and a nonaqueous electrolyte using the positive electrode; and initially charging the secondary battery assembly. When the secondary battery assembly is initially charged, a restraining pressure is determined based on the measured peel strength, and in a predetermined peel strength range, a higher restraining pressure is set for a secondary battery assembly including a positive electrode having a low peel strength than for a secondary battery assembly including a positive electrode having a large peel strength.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of application Ser. No. 15/811,021 filed Nov. 13,2017, which claims priority to Japanese Patent Application No.2016-227775 filed Nov. 24, 2016. The disclosure of the priorapplications is hereby incorporated by reference herein in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a method of producing a secondarybattery.

2. Description of Related Art

Nonaqueous electrolyte secondary batteries such as lithium ion secondarybatteries (lithium secondary batteries) are lighter in weight and have ahigher energy density than batteries of the related art. Therefore, theyhave recently been used as a so-called portable power supply for acomputer and a mobile terminal and a power supply for driving a vehicle.In particular, lightweight lithium ion secondary batteries capable ofobtaining a high energy density are expected to be increasingly appliedas high output power supplies for driving vehicles such as an electricvehicle (EV), a hybrid vehicle (HV), and a plug-in hybrid vehicle (PHV).

A typical configuration of a lithium ion secondary battery includes apositive electrode, a negative electrode, and a nonaqueous electrolyte.A typical configuration of the positive electrode includes a positiveelectrode current collector and a positive electrode active materiallayer provided on the positive electrode current collector. As apositive electrode active material contained in the positive electrodeactive material layer, a composite oxide containing lithium andmanganese such as a spinel type lithium and manganese composite oxideand a composite oxide in which another metal element is additionallyadded to a spinel type lithium and manganese composite oxide arefrequently used (for example, refer to Japanese Unexamined PatentApplication Publication No. 2002-298927 (JP 2002-298927 A)).

SUMMARY

The inventors conducted research and found that, when a secondarybattery is produced using a positive electrode including a positiveelectrode active material layer containing a lithium- andmanganese-containing composite oxide, if a secondary battery is producedusing a positive electrode having a low peel strength between a positiveelectrode active material layer and a positive electrode currentcollector, a capacity of the obtained secondary battery is low. The peelstrength between the positive electrode active material layer and thepositive electrode current collector is influenced by a mixing state ofa composition for forming a positive electrode active material layercontaining a positive electrode active material, a binder, and the like,and it is difficult completely control the mixing state. Therefore,produced positive electrodes may be mixed with positive electrodeshaving a low peel strength between the positive electrode activematerial layer and the positive electrode current collector. Currently,such a positive electrode having a low peel strength between thepositive electrode active material layer and the positive electrodecurrent collector has to be discarded. Therefore, there is room forimprovement in the yield of production of a secondary battery.

Here, the present disclosure provides a method through which it ispossible to produce a secondary battery with a high yield using apositive electrode including a positive electrode active material layercontaining a lithium- and manganese-containing composite oxide.

It is known that, in a secondary battery in which a composite oxidecontaining lithium and manganese (hereinafter referred to as a “lithium-and manganese-containing composite oxide”) is used as a positiveelectrode active material, manganese is eluted from the lithium- andmanganese-containing composite oxide, and thus the battery capacity isreduced. The inventors developed the idea that, in a positive electrodehaving a low peel strength between a positive electrode active materiallayer and a positive electrode current collector, there are relativelymany portions in which the positive electrode active material layer andthe positive electrode current collector are not in close contact witheach other, and thought that manganese is likely to be eluted from thelithium- and manganese-containing composite oxide in those portions, andthus completed the present disclosure.

A first method of producing a secondary battery disclosed here includesforming a positive electrode active material layer containing a lithium-and manganese-containing composite oxide on a positive electrode currentcollector to produce a positive electrode; measuring a peel strengthbetween the positive electrode active material layer and the positiveelectrode current collector; producing a secondary battery assemblyincluding the positive electrode, a negative electrode, and a nonaqueouselectrolyte using the positive electrode; and initially charging thesecondary battery assembly, wherein, when the secondary battery assemblyis initially charged, a charging rate is determined based on themeasured peel strength, and in a predetermined peel strength range, alower charging rate is set for a secondary battery assembly including apositive electrode having a low peel strength than for a secondarybattery assembly including a positive electrode having a large peelstrength.

In such a configuration, in the lithium ion secondary battery includinga positive electrode having a low peel strength between the positiveelectrode active material layer and the positive electrode currentcollector, when the charging rate is changed, it is possible to preventthe lithium- and manganese-containing composite oxide which is apositive electrode active material from being exposed to an acid(particularly, HF) generated during initial charging without increasingthe valence of manganese. Accordingly, it is possible to preventlow-valence manganese from being eluted from the lithium- andmanganese-containing composite oxide. Thus, it is also possible toproduce the secondary battery using a positive electrode having a lowpeel strength between the positive electrode active material layer andthe positive electrode current collector which was discarded in therelated art. Therefore, it is possible to reduce the number of positiveelectrodes to be discarded and as a result, it is possible to producethe secondary battery with a high yield.

In one form of the first method of producing a secondary batterydisclosed here, when the secondary battery assembly is initiallycharged, ultrasonic waves may be emitted toward an interface between thepositive electrode active material layer and the positive electrodecurrent collector, and the charging rate may be changed according to atransmission intensity of the ultrasonic waves.

In such a configuration, it is possible to further optimize the chargingrate in the initial charging process.

In one form of the first method of producing a secondary batterydisclosed here, when a transmission intensity of the ultrasonic waves isequal to or less than a predetermined value, the charging rate may bereduced.

In one form of the first method of producing a secondary batterydisclosed here, charging rate≤a×peel strength²+b, wherein a>0, b≈0.

In one form of the first method of producing a secondary batterydisclosed here, charging rate=a×peel strength²+b−c, wherein c may be avalue that satisfies a×peel strength²+b>c>0.

In one form of the first method of producing a secondary batterydisclosed here, charging rate={a×peel strength²+b}×c′, wherein c′ may bea value that satisfies 0<c′<1.

A second method of producing a secondary battery disclosed here includesforming a positive electrode active material layer containing a lithium-and manganese-containing composite oxide on a positive electrode currentcollector to produce a positive electrode; measuring a peel strengthbetween the positive electrode active material layer and the positiveelectrode current collector; producing a secondary battery assemblyincluding the positive electrode, a negative electrode, and a nonaqueouselectrolyte using the positive electrode; and performing initialcharging while restraining the secondary battery assembly, wherein, whenthe secondary battery assembly is initially charged, a restrainingpressure is determined based on the measured peel strength, and in apredetermined peel strength range, a higher restraining pressure is setfor a secondary battery assembly including a positive electrode having alow peel strength than for a secondary battery assembly including apositive electrode having a large peel strength.

In such a configuration, in the lithium ion secondary battery includinga positive electrode having a low peel strength between the positiveelectrode active material layer and the positive electrode currentcollector, when the restraining pressure is changed, it is possible toprevent the lithium- and manganese-containing composite oxide which is apositive electrode active material from being exposed to an acid(particularly, HF) generated during initial charging without increasingthe valence of manganese. Accordingly, it is possible to preventlow-valence manganese from being eluted from the lithium- andmanganese-containing composite oxide. Thus, it is also possible toproduce the secondary battery using a positive electrode having a lowpeel strength between the positive electrode active material layer andthe positive electrode current collector which was discarded in therelated art. Therefore, it is possible to reduce the number of positiveelectrodes to be discarded and as a result, it is possible to producethe secondary battery with a high yield.

In one form of the second method of producing a secondary batterydisclosed here, in the process of initial charging, ultrasonic waves maybe emitted toward an interface between the positive electrode activematerial layer and the positive electrode current collector, and therestraining pressure may be changed according to a transmissionintensity of the ultrasonic waves.

In such a configuration, it is possible to further optimize therestraining pressure in the initial charging process.

In one form of the second method of producing a secondary batterydisclosed here, when a transmission intensity of ultrasonic waves isequal to or less than a predetermined value, the restraining pressuremay be increased.

In one form of the second method of producing a secondary batterydisclosed here, restraining pressure≥d×peel strength+e, wherein d<0,e>0.

In one form of the second method of producing a secondary batterydisclosed here, restraining pressure=d×peel strength+e+f, wherein f>0.

In one form of the second method of producing a secondary batterydisclosed here, restraining pressure={d×peel strength+e}×f′, whereinf′>1.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a flowchart showing processes of a method of producing asecondary battery according to a first embodiment of the presentdisclosure;

FIG. 2 is a schematic diagram showing a configuration of a woundelectrode body of a lithium ion secondary battery obtained by a methodof producing a secondary battery according to first and secondembodiments of the present disclosure;

FIG. 3 is a sectional view schematically showing an internal structureof a lithium ion secondary battery obtained by the method of producing asecondary battery according to the first and second embodiments of thepresent disclosure;

FIG. 4 is a graph showing the relationship between a peel strength and acapacity retention rate of a lithium ion secondary battery of TestExample 1;

FIG. 5 is a graph showing correlations of a capacity retention rate witha peel strength and a charging rate during initial charging of thelithium ion secondary battery of Test Example 1;

FIG. 6 is a graph showing results obtained when a charging anddischarging rate of an assembly of the lithium ion secondary battery ofTest Example 1 is changed according to a transmission intensity ofultrasonic waves and when the charging and discharging rate is notchanged;

FIG. 7 is a flowchart showing processes of a method of producing asecondary battery according to the second embodiment of the presentdisclosure;

FIG. 8 is a graph showing the relationship between a peel strength and acapacity retention rate of a lithium ion battery with a restrainingpressure during initial charging of 15 kg/cm² with respect to anelectrode area and a lithium ion secondary battery with a restrainingpressure of 30 kg/cm²; and

FIG. 9 is a graph showing a correlation between a peel strength and arestraining pressure, and a capacity retention of a lithium ionsecondary battery of Test Example 2.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure will be described below withreference to the drawings. Here, components other than thoseparticularly mentioned in this specification that are necessary forimplementation of the present disclosure (for example, a generalconfiguration and a producing process of a secondary battery that do notcharacterize the present disclosure) can be recognized by those skilledin the art as design matters based on the related art in the field. Thepresent disclosure can be implemented based on content disclosed in thisspecification and common technical knowledge in the field. In addition,the sizes (a length, a width, a thickness, and the like) in the drawingsdo not reflect actual sizes.

Here, the term “secondary battery” in this specification refers to ageneral power storage device capable of repeatedly performing chargingand discharging, and is a term that includes a so-called storage batterysuch as a lithium ion secondary battery and a power storage element suchas an electric double-layer capacitor. The present disclosure will bedescribed below in detail with reference to embodiments, but thefollowing embodiments are not intended to limit the present disclosure.

First Embodiment

FIG. 1 shows processes of a method of producing a secondary batteryaccording to a first embodiment. The method of producing a secondarybattery according to the first embodiment shown in FIG. 1 includes aprocess (positive electrode producing process) S101 in which a positiveelectrode active material layer containing a lithium- andmanganese-containing composite oxide is formed on a positive electrodecurrent collector to produce a positive electrode, a process (peelstrength measuring process) S102 in which a peel strength between thepositive electrode active material layer and the positive electrodecurrent collector is measured, a process (battery assembly manufacturingprocess) S103 in which a secondary battery assembly including thepositive electrode, a negative electrode, and a nonaqueous electrolyteis produced using the positive electrode, and a process (initialcharging process) S104 in which the secondary battery assembly isinitially charged. In the initial charging process S104, a charging rateis determined based on the measured peel strength, and in apredetermined peel strength range, a lower charging rate is set for asecondary battery assembly including a positive electrode having a lowpeel strength than for a secondary battery assembly including a positiveelectrode having a large peel strength.

FIG. 2 is a diagram schematically showing a configuration of a woundelectrode body 20 of a lithium ion secondary battery 100 which is anexample of a secondary battery obtained by the production methodaccording to the present embodiment. FIG. 3 is a diagram schematicallyshowing an internal structure of the lithium ion secondary battery 100which is an example of a secondary battery obtained by the productionmethod according to the present embodiment.

First, the positive electrode producing process S101 will be described.In the positive electrode producing process S101, a positive electrodeactive material layer 54 containing a lithium- and manganese-containingcomposite oxide is formed on a positive electrode current collector 52to produce a positive electrode 50. The process S101 can be performedby, for example, applying a composition for forming a positive electrodeactive material layer, which contains a lithium- andmanganese-containing composite oxide, to the positive electrode currentcollector 52 and performing drying, and performing pressing asnecessary.

A type of the lithium- and manganese-containing composite oxide is notparticularly limited as long as it can function as a positive electrodeactive material. For example, a composite oxide represented byLi_(m)Mn_(x)M_(y)O_(n) (where m, x, y, and n satisfy 0.96≤m≤1.20,1.05≤x≤2.0, 0<y≤1.0, 2≤n≤4, and x>y, and M is at least one selected fromthe group consisting of Ni, Co, Ti, Fe, W, Cr, V, and Cu) as an averagecomposition can be used. The form of the lithium- andmanganese-containing composite oxide is not particularly limited, butlithium- and manganese-containing composite oxide particles aretypically used. When the lithium- and manganese-containing compositeoxide particles are used, an average particle size is generally 20 μm orless (typically 1 to 20 μm, for example, 5 to 15 μm). Here, the “averageparticle size” in this specification refers to a particle size (D₅₀,also referred to as a median diameter) corresponding to a cumulativefrequency of 50 volume % from the side of fine particles having a smallparticle size in a volume-based particle size distribution based on ageneral laser diffraction and light scattering method. As the positiveelectrode active material, a compound other than the lithium- andmanganese-containing composite oxide may be contained in the compositionfor forming a positive electrode active material layer. As the compound,a compound known as a positive electrode active material of a lithiumion secondary battery may be used. The positive electrode activematerial is contained preferably at greater than 50 mass %, morepreferably 80 mass % to 97 mass %, and most preferably 85 mass % to 96mass % with respect to the total solid content of the composition forforming a positive electrode active material layer.

The composition for forming a positive electrode active material layertypically contains a conductive material, a binder, and a solvent. As anexample of the conductive material, a carbon material such as carbonblack (for example, acetylene black, furnace black, and Ketchen black),coke, graphite (for example, natural graphite, and modified materialsthereof, and artificial graphite) may be exemplified. Among these,carbon black is preferable. The conductive material is contained atpreferably 0.1 to 20 mass %, and more preferably 1 to 15 mass % withrespect to the positive electrode active material. As an example of thebinder, polyvinylidene fluoride (PVdF), polyacrylate, polymethacrylate,and the like may be exemplified. The binder is contained at preferably0.5 to 10 mass % and more preferably 1 to 8 mass % with respect to thepositive electrode active material. As an example of the solvent, water,N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone, toluene, and the likemay be exemplified.

The composition for forming a positive electrode active material layermay contain a component other than the positive electrode activematerial, the conductive material, the binder, and the solvent. As anexample of the component, a thickener and the like may be exemplified.As an example of the thickener, a cellulosic polymer such ascarboxymethyl cellulose (CMC), methylcellulose, andhydroxypropylcellulose may be exemplified. Among these, carboxymethylcellulose is preferable. The thickener is contained at preferably 0.5 to10 mass % and more preferably 1 to 8 mass % with respect to the positiveelectrode active material.

The form of the composition for forming a positive electrode activematerial layer is not particularly limited as long as it can be appliedto the positive electrode current collector 52. A composition in theform of a paste, a slurry, an ink, a granulated substance or the likemay be used.

The composition for forming a positive electrode active material layercan be prepared by mixing the lithium- and manganese-containingcomposite oxide, the conductive material, the binder, and the solvent,and other components as necessary using a known mixing device. As themixing device, a planetary mixer, a homogenizer, a Clearmix, a Filmix, abead mill, a ball mill, a kneading extruder, and the like may beexemplified.

As the positive electrode current collector 52, a foil-like body made ofa metal having favorable conductivity (for example, aluminum, nickel,titanium, and stainless steel) can be appropriately used, and moreappropriately, an aluminum foil is used.

The composition for forming a positive electrode active material layeris applied to the positive electrode current collector according to aknown method. For example, a coating device such as a slit coater, a diecoater, a comma coater, a gravure coater, or a dip coater can be used toapply the composition for forming a positive electrode active materiallayer to the positive electrode current collector 52. Here, the positiveelectrode active material layer 54 may be formed only on one surface ofthe positive electrode current collector 52 or may be formed on bothsurfaces thereof, and preferably, formed on both surfaces thereof. Thus,the composition for forming a positive electrode active material layeris applied to one surface or both surfaces of the positive electrodecurrent collector 52, and preferably applied to both surfaces thereof.

After the application, drying is performed. Drying can be performed byremoving the solvent using a drying device such as a drying furnace. Adrying temperature and a drying time may be appropriately determinedaccording to a type and amount of the solvent included in the appliedcomposition for forming a positive electrode active material layer.

After the drying, pressing may be performed in order to adjust thethickness, the density, and the like of the positive electrode activematerial layer 54. Next, when the positive electrode current collector52 on which the positive electrode active material layer 54 is formed isprocessed to a predetermined size, it is possible to obtain the positiveelectrode (positive electrode sheet) 50 in which the positive electrodeactive material layer 54 is formed on the positive electrode currentcollector 52.

Next, the peel strength measuring process S102 will be described. In thepeel strength measuring process S102, a peel strength between thepositive electrode active material layer 54 and the positive electrodecurrent collector 52 is measured. When the peel strength is measured, aknown peel strength measurement method can be used. As an example of thepeel strength measurement method, a method in which a 90° peeling testis performed according to JISZ0237: 2009 (test method for adhesive tapesand adhesive sheets) may be exemplified. As a test piece provided tomeasure the peel strength, a cut out surplus piece generated whenprocessing into a predetermined size is performed in the positiveelectrode producing process S101 may be used. Alternatively, in thepositive electrode producing process S101, a size of a test piece isadded to a size of a desired positive electrode, a positive electrodecurrent collector in which a positive electrode active material layer isformed is produced, and the positive electrode and the test piece may beproduced during size processing. It is more appropriate to produce along positive electrode sheet roll that is great in length and collecttest pieces before and after or during a process of performing cuttinginto a positive electrode sheet 50 having a size corresponding to onesheet than to prepare a plurality of positive electrode sheets 50.

On the other hand, the battery assembly manufacturing process S103 isperformed. In the battery assembly manufacturing process S103, asecondary battery assembly including the positive electrode 50 obtainedin the positive electrode producing process S101; a negative electrode60, and a nonaqueous electrolyte (not shown) is produced. Here, theorder of performing the peel strength measuring process S102 and thebattery assembly manufacturing process S103 is not particularly limited.Either the peel strength measuring process S102 or the battery assemblymanufacturing process S103 may be performed first and both may beperformed at the same time.

The battery assembly manufacturing process S103 can be performedaccording to a general method. For example, the negative electrode 60,and a separator 70 are prepared, and the positive electrode 50 obtainedin the positive electrode producing process S101 is used to produce theelectrode body 20 shown in FIG. 2. Here, while a wound electrode body isproduced as the electrode body 20 in the present embodiment, the form ofthe electrode body 20 is not limited thereto, and the electrode body 20may be produced as a laminated electrode body according to a knownmethod.

The negative electrode 60 has typically a configuration in which anegative electrode active material layer 64 is provided on one surfaceor both surfaces (appropriately both surfaces) of a negative electrodecurrent collector 62. The negative electrode 60 can be produced byapplying a composition for forming a negative electrode active materiallayer, which contains a negative electrode active material, a thickener,a binder, and a solvent, to the negative electrode current collector 62,and performing drying, and performing pressing as necessary. As thenegative electrode active material, for example, a carbon material suchas graphite, hard carbon, and soft carbon may be used. As the binder,for example, styrene butadiene rubber (SBR) may be used. As thethickener, for example, carboxymethyl cellulose (CMC) may be used. Asthe solvent, water and the like may be used. As the negative electrodecurrent collector 62, a foil-like body made of a metal having favorableconductivity (for example, copper, nickel, titanium, and stainlesssteel) can be appropriately used, and more appropriately, a copper foilis used. Application of the composition for forming a negative electrodeactive material layer to the negative electrode current collector 62,drying, and any pressing operation can be performed according to a knownmethod.

As the separator 70, for example, a porous sheet (film) made of apolyethylene (PE), polypropylene (PP), polyester, cellulose, orpolyamide resin may be exemplified. Such a porous sheet may have asingle layer structure or a laminated structure having two or morelayers (for example, a three-layer structure in which a PP layer islaminated on both surfaces of a PE layer). A heat resistant layer (HRL)may be provided on a surface of the separator 70.

The wound electrode body 20 can be produced using the positive electrodesheet 50, the negative electrode sheet 60, and the separator 70according to a known method. For example, the wound electrode body 20can be produced by winding a laminate in which the positive electrodesheet 50 and the negative electrode sheet 60 are laminated with twoseparators 70 therebetween in a longitudinal direction, and performingpressing and squeezing in a lateral direction. In this case, as shown inFIG. 2, typically, a positive electrode active material layernon-forming portion 52 a provided at an end of the positive electrodesheet 50 in a width direction and a negative electrode active materiallayer non-forming portion 62 a provided at an end of the negativeelectrode sheet 60 in a width direction are laminated to protrude indirections opposite to each other and are wound. Accordingly, when thepositive electrode active material layer non-forming portion 52 a andthe negative electrode active material layer non-forming portion 62 aare integrated for power collection, it is possible to form theelectrode body 20 having favorable current collection efficiency. Here,the electrode body 20 may be produced by winding the laminate itself sothat the wound cross section becomes flattened.

Next, as shown in FIG. 3, the wound electrode body 20 is accommodated ina battery case 30 according to a known method. Specifically, a main bodyof the battery case 30, which has an opening, and a lid of the batterycase 30, which has an inlet for a nonaqueous electrolyte solution, areprepared. The lid has a size which closes the opening of the main bodyof the battery case 30. In addition, a thin-walled safety valve 36 witha configuration set such that, when an internal pressure of the batterycase 30 increases to a predetermined level or higher, the internalpressure is released, and an inlet (not shown) for injecting anonaqueous electrolyte solution are provided in the lid. For the batterycase 30, for example, a lightweight metal material having favorablethermal conductivity such as aluminum is used.

Next, a positive electrode terminal 42 and a positive electrode currentcollecting plate 42 a, and a negative electrode terminal 44 and anegative electrode current collecting plate 44 a are attached to the lidof the battery case 30. The positive electrode current collecting plate42 a and the negative electrode current collecting plate 44 a are weldedto the positive electrode current collector 52 and the negativeelectrode current collector 62 exposed to ends of the wound electrodebody 20, respectively, by ultrasonic welding or the like. Then, thewound electrode body 20 is accommodated in the battery case 30 from theopening of the main body of the battery case 30, and the main body andthe lid of the battery case 30 are welded by laser welding or the like.

Next, a nonaqueous electrolyte solution (not shown) is injected from theinlet of the lid of the battery case 30. After the nonaqueouselectrolyte solution is injected, the inlet is sealed, and thus it ispossible to obtain a lithium ion secondary battery assembly shown inFIG. 3. The same nonaqueous electrolyte solution as in a lithium ionsecondary battery of the related art can be used. Typically, anonaqueous electrolyte solution in which a supporting salt is includedin an organic solvent (nonaqueous solvent) can be used. As thenonaqueous solvent, an organic solvent such as various carbonates,ethers, esters, nitriles, sulfones, and lactones which are used in anelectrolyte solution of a general lithium ion secondary battery can beused without particular limitation. As a specific example, ethylenecarbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC),dimethyl carbonate (DMC), ethyl methyl carbonate (EMC),monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC),monofluoromethyl difluoromethyl carbonate (FDMC), trifluorodimethylcarbonate (TFDMC), and the like may be exemplified. Such nonaqueoussolvents can be used alone or in appropriate combination of two or morethereof. As the supporting salt, for example, a lithium salt such asLiPF₆, LiBF₄, and LiClO₄ (preferably LiPF₆) can be appropriately used. Aconcentration of the supporting salt is preferably 0.7 mol/L or more and1.3 mol/L or less. Here, the nonaqueous electrolyte solution may containvarious additives, for example, a gas generating agent, a film formingagent, a dispersant, and a thickener.

Next, the initial charging process S104 will be described. In theinitial charging process S104, the secondary battery assembly isinitially charged. In the initial charging process S104, a charging rateis determined based on the peel strength measured in the peel strengthmeasuring process S102, and in a predetermined peel strength range, alower charging rate is set for a secondary battery assembly including apositive electrode having a low peel strength than for a secondarybattery assembly including a positive electrode having a large peelstrength.

A specific example of determining a charging rate will be described withreference to Test Example 1. A battery of Test Example 1 was producedaccording to the following method. LiNi_(0.5)Mn_(1.5) O₄ having anaverage particle size of 10 μm as a lithium- and manganese-containingoxide, acetylene black (AB) as a conductive material, and polyvinylidenefluoride (PVDF) as a binder (mass ratio ofLiNi_(0.5)Mn_(1.5)O₄:AB:PVDF=87:10:3) were mixed with N-methylpyrrolidone (NMP) to prepare a slurry for forming a positive electrodeactive material layer. The slurry was applied to both surfaces of analuminum foil (positive electrode current collector) and dried, and thenprocessing to a predetermined size was performed to produce a positiveelectrode sheet. In this case, when a mixing time during slurrypreparation was adjusted in order to simulate variations in peelstrength during production, various positive electrode sheets havingdifferent peel strengths between the positive electrode active materiallayer and the positive electrode current collector were produced. Inaddition, when the positive electrode sheet was produced, a test piecefor measuring a peel strength was also produced. In addition, a naturalgraphite-based carbon material (C) having an average particle size of 20μm as a negative electrode active material, styrene butadiene rubber(SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener(mass ratio of C:SBR:CMC=98:1:1) were mixed with deionized water toprepare a slurry for forming a negative electrode active material layer.The slurry was applied to both surfaces of a copper foil and dried, andthen processing to a predetermined size was performed to produce anegative electrode sheet. In addition, a separator sheet (porouspolyolefin sheet) was prepared. The produced positive electrode sheetand negative electrode sheet, and two separator sheets were laminatedand wound to produce an electrode body. The produced electrode body towhich a current collector was attached was accommodated in a rectangularbattery case together with a nonaqueous electrolyte solution to obtain alithium ion secondary battery assembly. As the nonaqueous electrolytesolution, a solution in which LiPF₆ with a concentration of 1 mol/L wasdissolved in a solvent mixture containing monofluoroethylene carbonate(MFEC) and monofluoromethyl difluoromethyl carbonate (FDMC) (volumeratio 50:50) was used. The lithium ion secondary battery assembly wasrestrained, a restraining pressure of 15 kg/cm² was applied to anelectrode area, and initial charging was performed at various chargingrates. Then, discharging was performed from a full charged state, and adischarge capacity (initial capacity) was obtained. Thus, the lithiumion secondary battery of Test Example 1 was obtained. The lithium ionsecondary battery of Test Example 1 on which initial charging wasperformed was left in an environment at 60° C., and charging wasperformed at a constant current of 2 C up to 4.9 V and discharging wasperformed at a constant current of 2 C up to 3.5 V. This charging anddischarging was set as one cycle, which was performed over a total of200 cycles. Then, a battery capacity after charging and discharging over200 cycles was obtained (battery capacity after charging and dischargingover 200 cycles/initial capacity) and multiplied by 100 to obtain acapacity retention rate (%). On the other hand, a peel strength betweenthe positive electrode active material layer and the positive electrodecurrent collector of the positive electrode sheet used to produce thelithium ion secondary battery was measured using a 90° peeling testaccording to JISZ0237: 2009 (test method for adhesive tapes and adhesivesheets). Specifically, using a 90° peel testing machine(“SV-201-NA-50SL” commercially available from IMADA SEISAKUSHO Co.,Ltd.), the positive electrode active material layer was fixed to a teststand, the positive electrode current collector was pulled in adirection of 90°, and a peel strength was measured. The test wasperformed a plurality of times. The most frequent value was used as apeel strength between the positive electrode active material layer andthe positive electrode current collector of the positive electrodesheet. A peel strength of another positive electrode sheet was obtainedas a relative value when a peel strength of a positive electrode sheetobtained when mixing during slurry preparation was performed understandard conditions was set as 100.

FIG. 4 is a graph showing the relationship between a peel strength and acapacity retention rate of the lithium ion secondary battery of TestExample 1 on which, as initial charging, charging was performed at aconstant current of 0.2 C (a current value of ⅕C of a capacity estimatedfrom an amount of the positive electrode active material) up 10 to 4.9V, and constant voltage charging was then performed until the currentvalue reached 1/50C. As described above, when the peel strength is lowerthan a certain value, the capacity retention rate tends to decrease asthe peel strength decreases. In Test Example 1, when a cross section ofthe positive electrode sheet having a low peel strength and a crosssection of the positive electrode sheet having a large peel strengthwere observed under an electronic microscope, it was found that thepositive electrode sheet having a low peel strength had relatively manyportions in which the positive electrode active material layer and thepositive electrode current collector were not in close contact with eachother. In addition, in those portions, it was confirmed that a metal waseluted from LiNi_(0.5)Mn_(1.5)O₄, and it was suggested that manganesewas eluted from LiNi_(0.5)Mn_(1.5)O₄. Based on the above result, thereason for the above trend is inferred to be as follows. The positiveelectrode sheet having a low peel strength between the positiveelectrode active material layer and the positive electrode currentcollector had relatively many portions in which the positive electrodeactive material layer and the positive electrode current collector werenot in close contact each other. In those portions, no electrons weresupplied from the positive electrode current collector, and the valenceof manganese did not increase during initial charging, and exposure toan acid (particularly, HF) generated during initial charging occurred.Thus, manganese having a low valence was likely to be eluted from thelithium- and manganese-containing composite oxide.

FIG. 5 is a graph showing correlations of a capacity retention rate witha peel strength and a charging rate during initial charging. In FIG. 5,a test lithium ion secondary battery having a capacity retention ratethat is equal to or greater than an acceptance value (accepted product)is plotted as a series 1 (o in the graph), and a test lithium ionsecondary battery having a capacity retention rate that is less than anacceptance value (rejected product) is plotted as a series 2 (x in thegraph). As shown in FIG. 5, it can be seen that, as peel strengthdecreases, when the charging rate is reduced, the number of acceptedlithium ion secondary batteries increases. The reason for this isinferred to be as follows. In a lithium ion secondary battery assemblyincluding a positive electrode having a low peel strength, when thecharging rate is reduced, a lithium- and manganese-containing compositeoxide serving as a positive electrode active material is prevented frombeing exposed to an acid (particularly, HF) generated during initialcharging without increasing the valence of manganese. Therefore, it ispossible to prevent low-valence manganese from being eluted from thelithium- and manganese-containing composite oxide.

The dashed line in FIG. 5 represents an acceptance line of the peelstrength and the charging rate of the lithium ion secondary battery. Thedashed line in FIG. 5 can be approximated by charging rate=a×(peelstrength)²+b(a>0, b≈0).

Here, based on the findings obtained from Test Example 1, for example,the charging rate in the initial charging process S104 is determined tosatisfy charging rate≤a×(peel strength)²+b, (preferably, chargingrate<a×(peel strength)²+b). For example, a formula in which the chargingrate is slightly lower than that of the approximate formula of theacceptance line: charging rate=a×(peel strength)²+b−c (c is a value thatsatisfies a×(peel strength)²+b>c>0, for example, 0.05 to 0.15), orcharging rate={a×(peel strength)²+b}×c′ (c′ is a value that satisfies0<c′<1, for example, 0.75 to 0.90) is determined, a peel strength issubstituted into the formula, and thus the charging rate can bedetermined. According to such a method, in a predetermined peel strengthrange (particularly, in a range in which a peel strength is equal to orless than a predetermined value), a lower charging rate is set for alithium ion secondary battery assembly including a positive electrodehaving a low peel strength than for a lithium ion secondary batteryassembly including a positive electrode having a large peel strength.

Alternatively, based on the results in FIG. 5, a charging rate of aspecific value may be assigned for a specific range of a peel strengthsuch that initial charging is performed at a charging rate of 0.5 C whena peel strength is 80 or more, a charging rate of 0.2 C when a peelstrength is 60 or more and less than 80, and a charging rate of 0.1 Cwhen a peel strength is 40 or more and less than 60, and the chargingrate may be determined. In such a method also, in a predetermined peelstrength range (particularly, in a range in which a peel strength isequal to or less than a predetermined value), a lower charging rate isset for a lithium ion secondary battery assembly including a positiveelectrode having a low peel strength than for a lithium ion secondarybattery assembly including a positive electrode having a large peelstrength.

As described above, in the initial charging process S104, based on thetest results of the peel strength and the charging rate, it is possibleto determine a charging rate at which an accepted secondary battery canbe provided for the positive electrode 50 having a specific peelstrength. Specifically, when the relationship between the charging rateand the peel strength is formulated, the peel strength measured in thepeel strength measuring process S102 is substituted into the formula,and thus the charging rate can be determined. In addition, based on thetest results of the peel strength and the charging rate, a charging rateof a specific value is assigned for a specific range of a peel strength.Based on the assignment, the charging rate can be determined from thepeel strength measured in the peel strength measuring process S102.Here, a method of determining a charging rate is not limited to theabove method. Any method in which, in a predetermined peel strengthrange, a lower charging rate is set for a lithium ion secondary batteryassembly including a positive electrode having a low peel strength thanfor a lithium ion secondary battery assembly including a positiveelectrode having a large peel strength may be used. Here, a positiveelectrode of which the peel strength measured in the peel strengthmeasuring process S102 is too low may be discarded.

Initial charging can be performed such that, for example, an externalpower supply is connected between the positive electrode terminal 42 andthe negative electrode terminal 44 of the produced lithium ion secondarybattery assembly, and charging (typically, constant current charging) isperformed at the determined charging rate up to a predetermined voltage.For the initial charging, constant current charging may be performed atthe determined charging rate up to a predetermined voltage, and constantvoltage charging may be then performed up to another predeterminedvoltage. In the initial charging process S104, it is preferable that thesecondary battery assembly be restrained and initial charging beperformed while applying a restraining pressure.

Here, in the peel strength measuring process S102, the peel strength ismeasured using a test piece separate from the positive electrode 50.Thus, it is not possible to accurately measure the peel strength betweenthe positive electrode active material layer 54 and the positiveelectrode current collector 52 of the positive electrode 50 in somecases. In addition, after the peel strength measuring process S102, thepeel strength between the positive electrode active material layer 54and the positive electrode current collector 52 may be reduced due toimpact or the like. In addition, in the initial charging process S104,the peel strength between the positive electrode active material layer54 and the positive electrode current collector 52 may be lower than themeasurement value in the peel strength measuring process S102. Inaddition, during charging, the peel strength between the positiveelectrode active material layer 54 and the positive electrode currentcollector 52 may change. Therefore, in the initial charging processS104, ultrasonic waves may be emitted toward an interface between thepositive electrode active material layer 54 and the positive electrodecurrent collector 52, and the charging rate may be changed according toa transmission intensity of the ultrasonic waves.

When ultrasonic waves are emitted toward the interface between thepositive electrode active material layer 54 and the positive electrodecurrent collector 52, attenuation occurs when ultrasonic waves aretransmitted. An amount of attenuation is larger in a portion in whichthe positive electrode active material layer 54 and the positiveelectrode current collector 52 are not in close contact with each otherthan in a portion in which the positive electrode active material layer54 and the positive electrode current collector 52 are in close contactwith each other. Here, for example, when the transmission intensity ofultrasonic waves is equal to or less than a predetermined value, thecharging rate is reduced. The inventors compared two cases in whichconstant current charging at 0.2 C was performed for the lithium ionsecondary battery assembly of Test. Example 1 while scanning with anultrasonic probe, and when an area of a portion in which thetransmission intensity was 66% or less was greater than ⅕ of the totalelectrode area, the charging rate was lowered to 0.1 C and the chargingrate was not changed. The results are shown in FIG. 6. As shown in FIG.6, if the charging rate is reduced when the transmission intensity ofultrasonic waves is equal to or less than a predetermined value, it ispossible to obtain a lithium ion secondary battery having a highcapacity retention rate even when a positive electrodes having a peelstrength relative value of 60 and 40 (that is, the peel strength is low)is used. Therefore, in this manner, it is possible to further optimizethe charging rate in the initial charging process S104.

Emission of ultrasonic waves and measurement of the transmissionintensity can be performed according to known methods. For example,using a known ultrasonic inspection device (for example, refer toJapanese Unexamined Patent Application Publication No. 2015-215279 (JP2015-215279 A)) used for examination of a laminate seal, the batterycase 30 is provided between a transmitter and a receiver of theultrasonic inspection device, and ultrasonic waves can be emitted in adirection perpendicular to a lamination direction of the positiveelectrode 50 and the negative electrode 60.

After the initial charging process S104, an aging process may beperformed according to a known method. As described above, the lithiumion secondary battery 100 can be obtained. According to the productionmethod of the present embodiment, when the charging rate is selectedaccording to the peel strength between the positive electrode activematerial layer 54 and the positive electrode current collector 52, it isalso possible to produce the lithium ion secondary battery 100 includingthe positive electrode 50 having a low peel strength between thepositive electrode active material layer 54 and the positive electrodecurrent collector 52 which was discarded in the related art. Therefore,it is possible to reduce the number of positive electrodes 50 to bediscarded and as a result, it is possible to produce the lithium ionsecondary battery 100 with a high yield.

Second Embodiment

FIG. 7 shows processes of a method of producing a secondary batteryaccording to a second embodiment. The method of producing a secondarybattery according to the second embodiment shown in FIG. 7 includes athe process (positive electrode producing process) S101 in which apositive electrode active material layer containing a lithium- andmanganese-containing composite oxide is formed on a positive electrodecurrent collector to produce a positive electrode, a process (peelstrength measuring process) S102 in which a peel strength between thepositive electrode active material layer and the positive electrodecurrent collector is measured, a process (battery assembly manufacturingprocess) S103 in which a secondary battery assembly including thepositive electrode, a negative electrode, and a nonaqueous electrolyteis produced using the positive electrode, and a process (initialcharging process) S104′ in which initial charging is performed whilerestraining the secondary battery assembly. In the initial chargingprocess S104′, the restraining pressure is determined based on themeasured peel strength, and in a predetermined peel strength range, ahigher restraining pressure is set for a secondary battery assemblyincluding a positive electrode having a low peel strength than for asecondary battery assembly including a positive electrode having a largepeel strength.

Details of the positive electrode producing process S101, the peelstrength measuring process S102, and the battery assembly manufacturingprocess S103 performed in the second embodiment are the same as those inthe first embodiment, and descriptions thereof will be omitted.

The initial charging process S104′ of the second embodiment will bedescribed. In the initial charging process S104′, initial charging isperformed while restraining the secondary battery assembly. In theinitial charging process S104′, the restraining pressure is determinedbased on the peel strength measured in the peel strength measuringprocess S102, and in a predetermined peel strength range, a higherrestraining pressure is set for a secondary battery assembly including apositive electrode having a low peel strength than for a secondarybattery assembly including a positive electrode having a large peelstrength.

A specific example of determining a charging rate will be described withreference to Test Example 2. A battery of Test Example 2 was producedaccording to the following method. First, according to the same methodas in Test Example 1 described in the first embodiment, a lithium ionsecondary battery assembly was produced using a positive electrodehaving various peel strengths between a positive electrode activematerial layer and a positive electrode current collector.

The lithium ion secondary battery assembly was restrained, variousrestraining pressures were applied to an electrode area, constantcurrent charging was performed at a current value of 0.2 C (⅕C of acapacity estimated from an amount of a positive electrode activematerial) up to 4.9 V, and constant voltage charging was then performeduntil the current value reached 1/50C. Then, discharging was performedup to 3.5 V and a discharge capacity (initial capacity) was obtained.Thus, the lithium ion secondary battery of Test Example 2 was obtained.The lithium ion secondary battery of Test Example 2 on which initialcharging was performed was left in an environment at 60° C., andcharging was performed at a constant current of 2 C up to 4.9 V anddischarging was performed at a constant current of 2 C up to 3.5 V. Thischarging and discharging was set as one cycle, which was performed overa total of 200 cycles. Then, a battery capacity after charging anddischarging over 200 cycles was obtained (battery capacity aftercharging and discharging over 200 cycles/initial capacity) andmultiplied by 100 to obtain a capacity retention rate (%). On the otherhand, a peel strength between the positive electrode active materiallayer and the positive electrode current collector of the positiveelectrode sheet was obtained according to the same method as in TestExample 1. A peel strength of another positive electrode sheet wasobtained as a relative value when a peel strength of a positiveelectrode sheet obtained when mixing during slurry preparation wasperformed under standard conditions was set as 100.

FIG. 8 is a graph showing the relationship between a peel strength and acapacity retention rate of the lithium ion battery on which arestraining pressure during initial charging was 15 kg/cm² with respectto an electrode area and the lithium ion secondary battery on which arestraining pressure during initial charging was 30 kg/cm². As shown inFIG. 8, it can be understood that, in the lithium ion secondary batteryhaving a relative value of 60 and 40 (that is, the peel strength islow), the capacity retention rate is higher when the restrainingpressure is increased. The reason for this is inferred to be as follows.When the restraining pressure is increased, in a positive electrodesheet having a low peel strength between a positive electrode activematerial layer and a positive electrode current collector, it ispossible to reduce a portion in which the positive electrode activematerial layer and the positive electrode current collector are not inclose contact with each other which is a portion in which no electronsare supplied from the positive electrode current collector. As a result,a lithium- and manganese-containing composite oxide serving as apositive electrode active material is prevented from being exposed to anacid (particularly, HF) generated during initial charging withoutincreasing the valence of manganese. Therefore, it is possible toprevent low-valence manganese from being eluted from the lithium- andmanganese-containing composite oxide.

FIG. 9 is a graph showing correlations of a capacity retention rate witha peel strength and a restraining pressure. In FIG. 9, a test lithiumion secondary battery having a capacity retention rate that is equal toor greater than an acceptance value (here, 85%) (accepted product) isplotted as a series 1 (o in the graph), and a test lithium ion secondarybattery having a capacity retention rate that is less than an acceptancevalue (rejected product) is plotted as a series 2 ((x in the graph). Asshown in FIG. 9, it can be seen that, as the peel strength decreases,when the restraining pressure is higher, the number of accepted lithiumion secondary batteries increases.

The dashed line in FIG. 9 represents an acceptance line of the peelstrength and the charging rate of the lithium ion secondary battery. Thedashed line in FIG. 9 can be approximated by restrainingpressure=d×(peel strength)+e (d<0, e>0).

Here, based on the findings obtained from Test Example 2, for example,the restraining pressure in the initial charging process S104′ isdetermined to satisfy restraining pressure≥d×(peel strength)+e(preferably, restraining pressure>d×(peel strength)+e). For example, aformula in which the restraining pressure is slightly higher than thatof the approximate formula of the acceptance line: restrainingpressure=d×(peel strength)+e+f (f>0), or restraining pressure={d×(peelstrength)+e}×f′ (f′>1) is determined, a peel strength is substitutedinto the formula, and thus the restraining pressure can be determined.According to such a method, in a predetermined peel strength range(particularly, in a range in which a peel strength is equal to or lessthan a predetermined value), a higher restraining pressure is set for alithium ion secondary battery assembly including a positive electrodehaving a low peel strength than for a lithium ion secondary batteryassembly including a positive electrode having a large peel strength.

Alternatively, based on the results in FIG. 9, a restraining pressure ofa specific value may be assigned for a specific range of a peel strengthsuch that initial charging is performed at a restraining pressure of 15kg/cm² when a peel strength is 80 or more, a restraining pressure of22.5 kg/cm² when a peel strength is 60 or more and less than 80, and arestraining pressure of 30 kg/cm² when a peel strength is 40 or more andless than 60, and the restraining pressure may be determined. In such amethod also, in a predetermined peel strength range (particularly, in arange in which a peel strength is equal to or less than a predeterminedvalue), a higher restraining pressure is set for a lithium ion secondarybattery assembly including a positive electrode having a low peelstrength than for a lithium ion secondary battery assembly including apositive electrode having a large peel strength.

As described above, in the initial charging process S104′, based on thetest results of the peel strength and the restraining pressure, it ispossible to determine a restraining pressure at which an acceptedsecondary battery can be provided for the positive electrode 50 having aspecific peel strength. Specifically, when the relationship between therestraining pressure and the peel strength is formulated, the peelstrength measured in the peel strength measuring process S102 issubstituted into the formula, and thus the restraining pressure can bedetermined. In addition, based on the test results of the peel strengthand the restraining pressure, a restraining pressure of a specific valueis assigned for a specific range of a peel strength. Based on theassignment, the restraining pressure can be determined from the peelstrength measured in the peel strength measuring process S102. Here, amethod of determining a restraining pressure is not limited to the abovemethod. Any method in which, in a predetermined peel strength range, ahigher restraining pressure is set for a lithium ion secondary batteryassembly including a positive electrode having a low peel strength thanfor a lithium ion secondary battery assembly including a positiveelectrode having a large peel strength may be used. Here, a positiveelectrode of which the peel strength measured in the peel strengthmeasuring process S102 is too low may be discarded.

In the initial charging process S104′, restraining can be performedaccording to a known method. For example, a known restraining tool orthe like can be used for restraining. Initial charging can be performedsuch that, for example, an external power supply is connected betweenthe positive electrode terminal 42 and the negative electrode terminal44 of the produced lithium ion secondary battery assembly, and charging(typically, constant current charging) is performed to a predeterminedvoltage. For the initial charging, constant current charging may beperformed at the determined charging rate up to a predetermined voltage,and constant voltage charging may be then performed up to anotherpredetermined voltage.

Here, in the peel strength measuring process S102, the peel strength ismeasured using a test piece separate from the positive electrode 50.Thus, it is not possible to accurately measure the peel strength betweenthe positive electrode active material layer 54 and the positiveelectrode current collector 52 of the positive electrode 50 in somecases. In addition, after the peel strength measuring process S102, thepeel strength between the positive electrode active material layer 54and the positive electrode current collector 52 may be reduced due toimpact or the like. In addition, in the initial charging process S104′,the peel strength between the positive electrode active material layer54 and the positive electrode current collector 52 may be lower than themeasurement value in the peel strength measuring process S102. Inaddition, during charging, the peel strength between the positiveelectrode active material layer 54 and the positive electrode currentcollector 52 may change. Therefore, the initial charging process S104′,ultrasonic waves may be emitted toward an interface between the positiveelectrode active material layer and the positive electrode currentcollector, and the restraining pressure may be changed according to atransmission intensity of the ultrasonic waves.

When ultrasonic waves are emitted toward the interface between thepositive electrode active material layer 54 and the positive electrodecurrent collector 52, attenuation occurs when ultrasonic waves aretransmitted. An amount of attenuation is larger in a portion in whichthe positive electrode active material layer 54 and the positiveelectrode current collector 52 are not in close contact with each otherthan in a portion in which the positive electrode active material layer54 and the positive electrode current collector 52 are in close contactwith each other. Here, for example, when the transmission intensity ofultrasonic waves is equal to or less than a predetermined value, therestraining pressure is increased. Therefore, in this manner, it ispossible to further optimize the restraining pressure in the initialcharging process S104′.

After the initial charging process S104′, an aging process may beperformed according to a known method. As described above, the lithiumion secondary battery 100 can be obtained. According to the productionmethod of the present embodiment, when the restraining pressure isselected according to the peel strength between the positive electrodeactive material layer 54 and the positive electrode current collector52, it is also possible to produce the lithium ion secondary battery 100including the positive electrode 50 having a low peel strength betweenthe positive electrode active material layer 54 and the positiveelectrode current collector 52 which was discarded in the related art.Therefore, it is possible to reduce the number of positive electrodes 50to be discarded and as a result, it is possible to produce the lithiumion secondary battery 100 with a high yield.

Here, while the first embodiment and the second embodiment have beendescribed as separate forms, the method of producing a secondary batterydisclosed here can be performed in a combination of the first embodimentand the second embodiment. Specifically, both the charging rate and therestraining pressure may be changed according to the peel strengthmeasured in the peel strength measuring process.

The lithium ion secondary battery 100 obtained as described above can beused for various applications. Appropriate applications may include adriving power supply mounted in a vehicle such as an electric vehicle(EV), a hybrid vehicle (HV), and a plug-in hybrid vehicle (PHV).Typically, the lithium ion secondary battery 100 may be used in the formof an assembled battery in which a plurality of batteries are connectedin series and/or in parallel.

The rectangular lithium ion secondary battery 100 including a flat woundelectrode body has been exemplified above. However, the method ofproducing a secondary battery disclosed here can be applied forproduction of other types of lithium ion secondary battery. For example,the method can be applied for production of a lithium ion secondarybattery including a laminated electrode body. In addition, the methodcan be applied for production of a cylindrical lithium ion secondarybattery, a laminated lithium ion secondary battery, and the like. Inaddition, the method can be applied for production of a nonaqueouselectrolyte secondary battery other than a lithium ion secondarybattery.

While specific examples of the present disclosure have been describedabove in detail, these are only examples, and do not limit the scope ofthe claims. The technologies described in the claims include variousmodifications and alternations of the specific examples exemplifiedabove.

What is claimed is:
 1. A method of producing a secondary battery,comprising: forming a positive electrode active material layercontaining a lithium- and manganese-containing composite oxide on apositive electrode current collector to produce a positive electrode;measuring a peel strength between the positive electrode active materiallayer and the positive electrode current collector; producing asecondary battery assembly including the positive electrode, a negativeelectrode, and a nonaqueous electrolyte using the positive electrode;and performing initial charging while restraining the secondary batteryassembly, wherein, when the secondary battery assembly is initiallycharged, a restraining pressure is determined based on the measured peelstrength, and in a predetermined peel strength range, a higherrestraining pressure is set for a secondary battery assembly including apositive electrode having a low peel strength than for a secondarybattery assembly including a positive electrode having a large peelstrength.
 2. The method of producing a secondary battery according toclaim 1, wherein the initial charging of the secondary battery assemblyincludes: emitting ultrasonic waves toward an interface between thepositive electrode active material layer and the positive electrodecurrent collector, and changing the restraining pressure according to atransmission intensity of the ultrasonic waves.
 3. The method ofproducing a secondary battery according to claim 2, wherein therestraining pressure is increased when the transmission intensity of theultrasonic waves is equal to or less than a predetermined value.
 4. Themethod of producing a secondary battery according to claim 1, wherein:restraining pressure≥d×peel strength+e, and d<0, e>0.
 5. The method ofproducing a secondary battery according to claim 4, wherein: restrainingpressure=d×peel strength+e+f, and f>0.
 6. The method of producing asecondary battery according to claim 4, wherein: restrainingpressure={d×peel strength+e}×f′, and f′>1.